So far as the game currently goes, choosing Bella at the end of Season 2 blocks you from doing anything more than a kiss with Jill, though we do keep getting teased with the possibility of doing something with both Jill and Bella - so far, it's just teasing and hints that maybe something might happen someday, if we're very lucky.
As of the end of Episode 11, the choice you make at the end of Episode 8 is binding - if you choose Others, you lock out the five Main Girls; if you choose one of the Main Girl options (Josy and Maya, Bella, Jill, Sage) you lock out everyone else. If you do pick Others, then through Season 3, you keep seeing choices where you have to choose between two or more of them to continue with one and drop certain others. There are one or two exceptions - the big one is Zoey, who gives you the option to abandon your current path for hers, but there are also options to mildly cheat on your chosen path by flirting with various others, and one particular girl who's strongly hinted at having upcoming cheating options.